Conventionally, with the diversification of liquid crystal displays, various studies have been carried out on a low-pressure discharge lamp for a back light device in order to achieve a thin tube structure, a high luminance, a long life and the like. As one of the methods for achieving these, a method is known in which an electrode made from a material having a low work function such as nickel is formed in any of various shapes including the shapes of a bar, a cylinder, a bottomed cylinder, a cap and the like so as be decreased in size as much as possible. This method suppresses electrode consumption due to sputtering that takes place during lighting of a low-pressure discharge lamp.
For example, in the case of the cylindrical electrode described in JP 4 (1992)-137429 A, cathode glow discharge comes into the inner portion of the cylindrical electrode. Therefore, this suppresses a phenomenon in which a waste of an electrode material scattered by sputtering reaches portions of the inner wall of a low-pressure discharge lamp to cause blackening. Moreover, an electrode substance that has been sputtered is turned back into the electrode in the cylindrical electrode and reused. Therefore, mercury consumption resulting from electrode substance consumption also is suppressed. Thus, from the aspect of the performance of a low-pressure discharge lamp, it is effective to adopt a small-sized cylindrical electrode or the like.
However, in the case where the low-pressure discharge lamp is required to attain a higher luminance and thus is used in a large current region, and in the case where it is required that the low-pressure discharge lamp be of a thin tube structure and a smaller electrode be used in order to meet a demand for a size reduction of a liquid crystal display frame, the following further should be addressed.
That is, in the case where a smaller electrode is used and a lamp current is increased, a cathode glow discharge density (value obtained by dividing a current density per a unit effective discharge surface area of the electrode by a square of a sealing pressure of a rare gas) and a cathode fall voltage are increased so that a shortage of an effective discharge surface area of the electrode is compensated. This results in a phenomenon in which a glow discharge transition from normal glow to abnormal glow is caused. By the abnormal glow, the consumption of a rare gas sealed in a low-pressure discharge lamp is accelerated as a result of a rapid increase in the sputtering amount of an electrode material, thereby causing a problem of a short lamp life.
Furthermore, the use of a thin tube structure and a large current density, and a reduction in space for a low-pressure discharge lamp unit cause an atmospheric temperature during lighting of a low-pressure discharge lamp to be increased excessively to a temperature not lower than a temperature at which an optimum level of an emitted luminous flux is maintained, thereby also causing a problem of a decrease in an emitted luminous flux.